1. Field of the Invention
The present invention relates to an optical device and method of fabricating the same, and more particularly to a reverse mesa ridge waveguide type laser diode and method of fabricating the same.
2. Discussion of Related Art
A laser diode, which receives current and outputs laser light, is used as an optical signal generation source in optical communications systems, and as a light source for an instrumentation equipment, information processing apparatus and pointer.
FIG. 1 is a cross-sectional view showing a conventional forward mesa ridge waveguide type laser diode. As shown in FIG. 1, a N-type buffer layer 2, a active layer 3, a P-type waveguide layer 4, an etch stop layer 5, a P-type clad layer 6 and a P-type contact layer 7 are sequentially formed on a N-type substrate 1 by metal organic chemical vapor deposition (MOCVD) technique. The contact layer 7 and the clad layer 6 are etched to form a forward mesa ridge. An oxide layer 8 acting as a protection layer is formed on the entire surface of the substrate uniformly so as to expose the upper surface of the forward mesa ridge. A P-type ohmic metal layer 9 is formed on the substrate such that it contacts with the exposed surface of the forward mesa ridge. A N-type ohmic metal layer 10 is then formed beneath the substrate 1.
Although it is easy to fabricate the above-described forward mesa ridge waveguide type laser diode, the laser of multiple mode may be generated from it due to its wide waveguide width. To overcome this problem, a reverse mesa ridge waveguide type laser diode has been proposed.
FIG. 2 is a cross-sectional view showing the conventional reverse mesa ridge waveguide type laser diode.
As shown in FIG. 2, a N-type buffer layer 12, an active layer 13, a P-type waveguide layer 14, an etch stop layer 15, a P-type clad layer 16 and a P-type contact layer 17 are sequentially formed on a N-type substrate 11 by MOCVD technique. The contact layer 17 and the clad layer 16 are etched to form a reverse mesa ridge. An oxide layer 10 acting as a protection layer is formed on the substrate uniformly so as to expose the upper surface of the reverse mesa ridge. A polyimide layer 19 is filled in the etched portions at both sides of the reverse mesa ridge. A P-type ohmic metal layer 18 is formed on the entire surface of the substrate such that it contacts with the exposed surface of the reverse mesa ridge. A N-type ohmic metal layer 21 is formed beneath the substrate 11.
Since the waveguide width in the laser diode as shown in FIG. 2 is narrow due to the reverse mesa ridge, it is possible to generate a single-mode laser. Furthermore, the contact resistance and serial resistance are reduced due to the wide upper width of the ridge. However, to decrease the waveguide width with increasing the upper width of the ridge, it is preferable that the clad layer 16 is thick. In result, the laser diode as showing in FIG. 2 has a serial resistance higher than that of a planar buried heterostructure (PBH) laser diode.